FIG. 1 depicts VNA 100 according to a conventional design. As shown in FIG. 1, VNA 100 comprises switch 101 to switch between VNA ports A through D to establish a path to a single reference receiver 102. When the RF signal path switch 101 changes position, the termination of the test port also changes. The change in termination causes the source match term to be different from the load match term. The difference is referred to as the “switch error.” The standard twelve term VNA error model derives the load match term from the through connection. Additionally, N port VNA calibration methods require a minimum of N−1 paths to be characterized. For a four-port VNA, three through paths are characterized through calibration procedures. However, most N-port devices use connector combinations that do not allow the use of a flush through connection required in typical short, open, load, through (SOLT) calibration method, Accordingly, adapter removal calibration methods are usually employed. This requires another extra calibration step.
The “unknown thru” calibration is ideally suitable to calibrate VNAs to test devices with non mate-able connector combinations. Like the TRL family of calibrations, it is based on the eight term error model and requires two receivers for each test port to obtain the necessary data to determine the VNA's systematic error terms. Accordingly, the usual implementation of the unknown thru calibration method cannot be applied to VNA 100. For VNA's that only possess a single reference receiver, calibration can be unduly time consuming.